This invention relates generally to apparatus for logging earth boreholes and more specifically to methods and apparatus which utilize means in addition to gravity for assisting the well logging instrument in traversing deviated earth boreholes.
It has become relatively common within the last few years to drill wells in search of oil and gas and the like with a portion of the bore deviating from the usual vertical orientation thereof. The deviation or inclination may extend for a considerable distance, sometimes returning to the usual vertical orientation. In some instances, such boreholes may extend past 90 degrees from the vertical and actually be extending in the up direction for some distance.
It is well known in the art of drilling such wells to attempt the logging of the formations surrounding such boreholes with logging instruments run into the well bore on a wireline and/or a cable to perform various operations. Such tools usually depend upon the force of gravity to permit positioning of the well tools at the desired formation within the well bore.
Manifestly, the relatively horizontal angle of the deviated portion of the well bore will not permit the wireline actuated tools, to move into the lower portion of the well bore since friction of the logging tool in the deviated portion works against the force of gravity. Thus it has become essential to provide some means of causing the well logging instrument to pass through the deviated portions of the well bore.
An additional problem commonly associated with such boreholes relates to the instability of some formations penetrated by the well bore, which results in borehole diameter changes, some of which are very abrupt. Ledges are thus formed and the logging instrument lodges against these ledges.
Another problem exists in a deviated borehole when the cable used to raise and lower the logging cable becomes "key seated". The term "key seated" refers to the situation where, due to well bore deviation or passing over a ledge within the borehole, the logging cable wears a groove or slot in the ledge. The friction caused by the logging cable passing within the groove makes it appear from surface indications that the downhole logging instrument is lodged within the borehole. Further compounding the problem is the fact that since the cable, not the logging instrument, is the source of friction the cable cannot be freed by "pulling loose". "Pulling loose" consists of exerting sufficient force on the cable from the surface to separate the cable from the instrument at the connection point between the two. The successful procedure results in the loss of the instrument but allows retrieval of the cable to the surface. The instrument can later be recovered by an operation termed "fishing" which is well known in the art of well drilling operations.
Yet another problem encountered when a cable becomes "key seated" occurs when the instrument is being removed or upwardly traversing the borehole. The instrument will become lodged at the point of "key seating", its upper portion actually attempting to pass into the groove created by the cable passing over the formation.
Thus, it has proven difficult to adequately log the earth formations surrounding these deviated sections utilizing only gravitational force for descent. While some prior art methods have addressed the problem of assisting the downward traverse of the instrument through the borehole, none have also addressed the further problem of aiding the ascent of the logging instrument.